1. Field of the Invention
The present invention relates to a direct injection internal combustion engine of a compression ignition type in which a piston top surface is formed with a cavity so that fuel may be directly injected into the cavity.
2. Description of the Prior Art
A direct injection internal combustion engine of the compression ignition type, in which a piston top surface is formed with a recess (which will be termined as a "cavity") to thereby form a combustion chamber, is frequently used as a large-sized engine because it is advantageous over a compression ignition type internal combustion engine having a swirl chamber and a pre-combustion chamber, in that it has no communication channel between the main and auxiliary chambers of the combustion chambers and can have a relatively low compression ratio.
However, small-sized engines having small cylinder diameters suffer more from problems in the formation of the air-fuel mixture than do the large-sized engines.
In the direct injection internal combustion engine of a compression ignition type according to the prior art, more specifically, a fuel injection nozzle is arranged, as shown in FIG. 1, generally at the center of cavity C formed in the top surface of a piston P so that a plurality of fuel sprays may be radially injected from a plurality of injection ports. The swirling flow (or swirl), which has been generated by the port of the intake valve during the suction stroke of the engine, still resides even at the end of the compression stroke, so that it prepares the mixture while swirling the fuel spray (in the direction of the arrow in FIG. 1) within the cavity C, as shown. The diameter of cavities generally used is within the range of 40 to 70% that of the piston P or the cylinder. In a small-sized engine in which the piston P has a diameter of not more than 100 mm, the diameter of the cavity C is small and is reduced to a smaller value if it is intended to further enlarge the compression ratio. As a result, the fuel spray, which has been radially injected from the plural injection ports of the fuel injection nozzle, impinges upon the inner wall surface of the cavity C, as shown in FIG. 2, to wet the wall surface in the form of a liquid film or coarse droplets so that it will not be effectively burned. As a result, the mixture effective for combustion is reduced to invite problems such as a reduction in the output power and mileage or the generation of smoke.
In order to prevent the fuel from impinging upon the cavity wall surface, there are generally adopted a method (a), by which the swirling flow to be generated in the combustion chamber is intensified; a method (b), by which the fuel injection nozzle has its injection ports reduced in size but increased in number; and a method (c), by which the compression ratio is increased to raise the pressure (or air density) in the cavity C at the timing of the fuel injections so that the spray penetration of the fuel injection nozzle may be weakened.
According to method (a), in the engine having a cylinder diameter of 100 to 120 mm, the swirl ratio (which is a measure of the intensity of the swirl to be generated in the combustion chamber; swirl velocity/mean piston speed) is about 4, and is limited to about 3.5 to 3.6 for an engine having a cylinder diameter not larger than 90 mm. If the swirl ratio is made more than the above-specified value, on the contrary, there arises a problem in that the resistance of the intake port to the air is augmented to remarkably degrade the volumetric efficiency of the engine.
In method (b), if the injection port of the fuel injection nozzle is made small, the fuel is atomized to weaken its penetration, but the injection port is liable to be clogged, if it is made too small, so that a practical problem is raised for injection ports having a size not larger than 0.15 mm. If the number of the injection ports is increased, on the other hand, the sprays having been injected from adjoining injection ports merge in the vicinity of the side wall of the cavity C (as hatched in FIG. 1), as shown by a broken line, thereby raising a problem in that an overrich region of the fuel is locally formed to cause the smoking phenomenon. In an engine having a cylinder diameter not smaller than 120 mm, the number of the injection ports is generally 4 or 5.
In method (c), the compression ratio is determined by the ratio between the whole clearance volume at bottom dead center and the clearance volume at top dead center. In the direct injection internal combustion of the compression ignition type therefore, the volume of the cavity has to be not less than 70% of the clearance volume so that a sufficient output power may be generated. In order to increase the compression ratio, therefore, the clearance between the cylinder head and the piston top surface as is not contributable to combustion is desirably made as small as possible. However, the above-specified clearance has a lower limit of about 0.5 mm if the thermal expansions of the engine parts due to combustion and product errors are taken into consideration. As a result, there arises a problem in that the compression ratio cannot be made higher for the smaller engine, or that the assembly and adjustment of the engine become difficult if the compression ratio used is high.
We, the Inventors, have succeeded in reaching the present invention by repeating systematic experiments, analyses and trials so that the problems concomitant with the small-sized the direct injection internal combustion engine of compression ignition type thus far described in the prior art might be solved.
From the experiments and analyses mentioned above, we have succeeded in obtaining the following findings.
The conditions of the direct injection internal combustion engine of the compression ignition type of the prior art for preparing the air-fuel mixture had to satisfy the following four items.